The invention concerns a method for routing data frames, for example Ethernet frames, in an Ethernet routing device. The invention applies to DSL modems, but is not limited to that environment.
IP multicasting is the transmission of an IP datagram to a group of devices identified by a single IP destination address. A device connected to a LAN (Local Area Network), for example an Ethernet LAN, may join or leave a group, i.e. request that multicast packets concerning a certain group be addressed to it or not. The Internet Group Management Protocol (‘IGMP’) provides a way for a device to report its multicast group membership to adjacent routers.
The LAN can be connected to a WAN (Wide Area Network) through a routing device. This routing device, which may be an ADSL modem, has to learn which packets received on the WAN are to be forwarded on the LAN in the context of multicast groups. For this purpose, it can monitor IGMP packets sent from the devices on the LAN to the WAN routers. This process, called ‘snooping’, allows the routing device to update its internal tables for the purpose of filtering packets from the WAN.
The number of IGMP packets may be fairly important. If a switch module connected to the LAN communicates IGMP packets to the multicast management control module of the routing device through control busses, the bandwidth limitations might be too important. On the other hand, the processing of frames—IGMP or not—should not be made more complex simply because of any special treatment of the IGMP frames.